Throughout this application, various publications are referenced in parentheses by author and year. Full citations for these references may be found at the end of the specification immediately preceding the claims. The disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this invention pertains.
The contraction of striated muscle is initiated when calcium (Ca2+) is released from tubules within the muscle cell known as the sarcoplasmic reticulum (SR). Calcium release channels, called ryanodine receptors (RyR), on the SR are required for excitation-contraction (EC) coupling. The type 2 ryanodine receptor (RyR2) is found in the heart, while the type 1 ryanodine receptor (RyR1) is found in skeletal muscle. The RyR1 receptor is a tetramer comprised of four 565,000 dalton RyR1 polypeptides and four 12,000 dalton FK-506 binding proteins (FKBP12). FKBP12s are regulatory subunits that stabilize RyR channel function (Brillantes et al., 1994) and facilitate coupled gating between neighboring RyR channels (Marx et al., 1998); the latter are packed into dense arrays in specialized SR regions that release intracellular stores of Ca2+, thereby triggering muscle contraction. In addition to FKBP12, the RyR1 macromolecular complex also includes the catalytic and regulatory subunits of PKA, and the phosphatase PP1 (Marx et al., 2001).
One FKBP12 molecule is bound to each RyR1 subunit. Dissociation of FKBP12 significantly alters the biophysical properties of the channels, resulting in the appearance of subconductance states, and increased open probability (Po) of the channels (Brillantes et al., 1994; Gaburjakova et al., 2001). In addition, dissociation of FKBP12 from RyR1 channels inhibits coupled gating resulting in channels that gate stochastically rather than as an ensemble (Marx et al., 1998). Coupled gating of arrays of RyR channels is thought to be important for efficient EC coupling that regulates muscle contraction (Marx et al., 1998). FKBPs are cis-trans peptidyl-prolyl isomerases that are widely expressed and subserve a variety of cellular functions (Marks, 1996). FKBP12s are tightly bound to and regulate the function of the skeletal (RyR1) (Brillantes et al., 1994; Jayaraman et al., 1992) and cardiac (RyR2) (Kaftan et al., 1996) muscle Ca2+ release channels, as well as a related intracellular Ca2+ release channel known as the type 1 inositol 1,4,5-triphosphate receptor (IP3R1) (Cameron et al., 1997), and the type I transforming growth factor β (TGFβ) receptor (TβRI) (Chen et al., 1997).
Heart failure (HF) is a leading cause of mortality and morbidity world-wide. The disease is characterized by a progressive decrease of contractile function of the heart that leads to hypoperfusion of critical organs. Moreover, many patients with moderate degrees of cardiac dysfunction have substantial impairment of exercise capacity that cannot be explained solely by the extent of their HF (Harrington and Coats, 1997; Minotti et al., 1991; Sullivan and Hawthorne, 1995; Wilson, 1995). This has led to the hypothesis that a primary skeletal muscle defect exists in patients with HF in addition to a primary cardiac muscle defect (Minotti et al., 1991).
The majority of HF patients experience a progressive deterioration of quality-of-life due to their significantly reduced exercise tolerance. In the more severe cases of HF (New York Heart Association class IV), the 2-year mortality rate is over 50% (Braunwald, 1992). Patients and animal models of HF are characterized by a maladaptive response that includes chronic hyperadrenergic stimulation (Bristow et al., 1982). The pathogenic significance of chronic hyperadrenergic stimulation in HF is supported by therapeutic strategies that decrease β-adrenergic stimulation and left ventricular myocardial wall stress and potently reverse ventricular remodeling (Barbone et al., 2001; Bristow et al., 1996).
Chronic β-adrenergic stimulation during HF is associated with cAMP-dependent protein kinase (PKA) hyperphosphorylation of cardiac RyR2 receptors (Marx et al., 2000). The consequent defective function of RyR2, manifested as an SR Ca2+ leak through the PKA-hyperphosphorylated channel, has been proposed as a contributing factor to depressed contractile function and arrhythmogenesis in HF (Marks et al., 2002; Marx et al., 2000). Consistent with this hypothesis, PKA hyperphosphorylation of RyR2 in failing hearts has been demonstrated in vivo in both animal models and in patients with HF undergoing cardiac transplantation (Antos et al., 2001; Marx et al., 2000; Ono et al., 2000; Reiken et al., 2001; Semsarian et al., 2002; Yano et al., 2000).
Using animal models of HF, depressed contractile function, including accelerated fatigue development, has been demonstrated in both skeletal muscle (Lunde et al., 2001; Lunde et al., 2002; Perreault et al., 1993) and diaphragmatic muscle (MacFarlane et al., 2000). In these studies, a defect in Ca2+ signaling was proposed, but the molecular mechanism underlying impaired skeletal muscle function in HF has hitherto not been elucidated.